std::format_kind
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C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Text processing library
Regular expressions library (C++11)
Formatting library (C++20)
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
(C++26)
Formatting library
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)(C++20)(C++20)
(C++26)
(C++23)
Formatter
(C++20)
(C++23)
(C++23)
(C++23)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
(C++23)
format_kind
(C++23)
Formatting arguments
(C++20)
(C++20)
(C++20)(C++20)(C++20)
(C++20) (deprecated in C++26)
(C++20)(C++20)
(C++20)
Defined in header
<format>
template< class R >
constexpr /* unspecified */ format_kind = /* unspecified */;
(1)
(since C++23)
constexpr /* unspecified */ format_kind = /* unspecified */;
template< ranges::input_range R >
(2)
(since C++23)
requires std::same_as <R, std::remove_cvref_t <R>>
The variable template format_kind selects an appropriate std::range_format for a range R.
std::format_kind<R> is defined as follows:
- If std::same_as <std::remove_cvref_t <ranges::range_reference_t <R>>, R> is true, std::format_kind<R> is std::range_format::disabled.
- Otherwise, if
R::key_typeis valid and denotes a type:- If
R::mapped_typeis valid and denotes a type, letUbe std::remove_cvref_t <ranges::range_reference_t <R>>.
- If
- If either
Uis a specialization of std::pair orUis a specialization of std::tuple and std::tuple_size_v <U> == 2, std::format_kind<R> is std::range_format::map.
- Otherwise, std::format_kind<R> is std::range_format::set.
- If either
- Otherwise, std::format_kind<R> is std::range_format::sequence.
A program that instantiates the primary template of the format_kind variable template is ill-formed.
Given a cv-unqualified program-defined type T that models input_range, a program can specialize format_kind for T. Such specializations are usable in constant expressions, and have type const std::range_format .
[edit] Possible implementation
namespace detail { template< typename > constexpr bool is_pair_or_tuple_2 = false; template< typename T, typename U > constexpr bool is_pair_or_tuple_2<std::pair <T, U>> = true; template< typename T, typename U > constexpr bool is_pair_or_tuple_2<std::tuple <T, U>> = true; template < typename T > requires std::is_reference_v <T> || std::is_const_v <T> constexpr bool is_pair_or_tuple_2<T> = is_pair_or_tuple_2<std::remove_cvref_t <T>>; } template< class R > constexpr range_format format_kind = [] { static_assert(false, "instantiating a primary template is not allowed"); return range_format::disabled; }(); template< ranges::input_range R > requires std::same_as <R, std::remove_cvref_t <R>> constexpr range_format format_kind<R> = [] { if constexpr (std::same_as <std::remove_cvref_t <std::ranges::range_reference_t <R>>, R>) return range_format::disabled; else if constexpr (requires { typename R::key_type; }) { if constexpr (requires { typename R::mapped_type; } && detail::is_pair_or_tuple_2<std::ranges::range_reference_t <R>>) return range_format::map; else return range_format::set; } else return range_format::sequence; }();
[edit] Example
Run this code
#include <filesystem> #include <format> #include <map> #include <set> #include <vector> struct A {}; static_assert(std::format_kind<std::vector <int>> == std::range_format::sequence); static_assert(std::format_kind<std::map <int, int>> == std::range_format::map); static_assert(std::format_kind<std::set <int>> == std::range_format::set); static_assert(std::format_kind<std::filesystem::path > == std::range_format::disabled); // ill-formed: // static_assert(std::format_kind<A> == std::range_format::disabled); int main() {}